1. The Field of the Invention
The present invention relates to upflow bioreactors for digesting biodegradable materials. More particularly, the present invention relates to methods for digesting biodegradable materials that contain an amount of solid, non-digestible matter.
2. The Relevant Technology
A bioreactor is a device that uses bacteria to promote the decomposition, biodegradation, or “digestion” of organic waste materials into simple organics and gaseous biogas products. Biogas is typically a mixture of methane, carbon dioxide, hydrogen sulfide, and other volatile organic compounds. If produced in sufficient quantities, the methane gas can be used as a fuel. A bioreactor can also be used to treat and detoxify organic waste matter and wastewater.
Anaerobic digestion is a traditional method used to treat wastewater containing high concentrations of organic matter. Anaerobic digestion removes large quantities of organic matter from the waste material and produces biogas as a useful byproduct. Anaerobic digestion is particularly suitable for wastewater containing high concentrations of organics, such as wastewater generated through agricultural production and processing.
Many attempts have been made to decompose organic waste using closed vessels. One type of closed vessel reactor that has shown high decomposition rates is the upflow anaerobic sludge blanket reactor. In this reactor, waste material is introduced into the bottom of the reactor and forced up through the vessel where it passes through a blanket of bacteria, which decompose the organic material to produce biogas that can be collected and used as a fuel.
To achieve high decomposition rates in an upflow bioreactor, the bacterial culture should be well established. Establishing the bacterial culture can be time consuming. It typically requires up to six-months to start up a new reactor and establish the bacterial culture. Nevertheless, one important advantage of an upflow bioreactor is that it can be operated continuously. Thus, once the bacterial culture is established, the high rate of digestion can be maintained for an extended period of time (e.g. months or even years).
Such continuous operation is, however, problematic in situations where the waste stream contains a high proportion of non-digestible inorganic matter, such as sand and organic matter that is not readily digested, such as hair or sawdust. The materials tend to collect in the in the digester over time and reduce the efficiency of the digester.